In recent years, plant growth using artificial light sources has been the subject of much research. In particular, cultivation methods using illumination from light emitting diodes (LED), which exhibit excellent monochromaticity, provide favorable energy conservation and long life, and can be easily miniaturized, are garnering much attention. Based on the results of research to date, one emission wavelength band that has been confirmed as an effective light source for plant growth (photosynthesis) is red light having a wavelength within the region from 600 to 700 nm. Light within the wavelength vicinity of 660 to 670 nm exhibits particularly favorable reaction efficacy for photosynthesis, and is thus a preferred light source. Examples of conventional red light emitting diodes of this wavelength that have been investigated include those having light emitting layers composed of AlGaAs and InGaNP and the like, but a high-output light emitting diode has yet to be achieved (for example, see Patent Documents 1 to 3).
On the other hand, compound semiconductor LEDs having a light emitting layer composed of an aluminum-gallium-indium phosphide (composition formula: (AlXGa1-X)YIn1-YP, wherein 0≦X≦1 and 0≦Y≦1) are also known. Among these LEDs, a light emitting layer having the composition Ga0.5In0.5P exhibits the longest wavelength, and the peak wavelength obtained from this light emitting layer is in the vicinity of 650 nm. Accordingly, achieving practical application of, and a high level of brightness for, compound semiconductor LEDs in the region to the long wavelength side of 655 nm has proven problematic.
Further, a light emitting unit having a light emitting layer composed of (AlXGa1-X)YIn1-YP (wherein 0≦X≦1 and 0≦Y≦1) is generally formed on a monocrystalline substrate of gallium arsenide (GaAs), which is optically opaque to the light emitted from the light emitting layer and is not particularly strong mechanically.
Accordingly, much research is being conducted with the aims of obtaining higher brightness visible light LEDs and achieving further improvements in the mechanical strength of devices. In other words, techniques have recently been disclosed in which the opaque substrate material such as GaAs is removed, and a support layer that transmits the emitted light and is composed of a transparent material that exhibits superior mechanical strength to conventional materials is then bonded to form a junction LED (for example, see Patent Document 4). On the other hand, investigations have also been conducted, for laser devices having a different light emission mechanism, into light emitting layers having strain, but there are currently no practical applications of strained light emitting layers in the field of light emitting diodes (for example, see Patent Document 5).
Furthermore, investigations are also being pursued into light emitting diode light emitting units that utilize a quantum well structure. However, because the quantum effect obtained by utilizing a quantum well structure shortens the emission wavelength, this effect has been unable to be applied to techniques requiring wavelength lengthening (for example, see Patent Document 6).